The general technical problem aimed at by the present invention is that of the adaptation of a data transmission to the fluctuations of the propagation channel. The so-called Adaptive Coding and Modulation technique, known by the acronym ACM, which consists in adapting the modulation, coding pair as a function of an item of information about the quality of the propagation channel, is known. This technique is notably used in the Wimax, UMTS, 3GPP/LTE or else DVB-S2 standards. The objective of this method is to determine the best modulation and coding parameters to guarantee a given level of quality of service while optimizing the useful bitrate offered by the system.
The implementation of the ACM method in a transmission system requires the management of several types of modulation and several rates of one and the same correcting code. In order for the bitrate/protection optimization to be as fine as possible and to allow efficient adaptation to the variations of the propagation channel, it is necessary in the absolute to benefit from a large possible number of modulation, coding pairs. Now, this necessity comes up against technological limits since it involves having significant resources in terms of memory space so as notably to store the various charts making it possible to deduce the probability of error associated with a modulation/coding pair as a function of the signal-to-noise ratio measured on the channel or of some other equivalent metric. Furthermore, too large a number of possible choices can also give rise to instability of the control loop which, on the basis of a measurement of the quality of the transmission link, carries out the adaptation of the modulation and coding parameters and thus cause lags in the processings carried out which may be penalizing in a real-time context.
The actual implementation of the ACM method is therefore generally performed by way of a limited number of modulation/coding pairs.
This significant granularity then presents a drawback for the performance of the system. Indeed, the application of the ACM method is aimed at determining the modulation/coding pair which makes it possible to comply with a given constraint of error rate at reception. However, the choice adopted to attain this objective causes a decrease in the useful bitrate and in the spectral efficiency on account of the redundancy added by the correcting code. The switchover from one modulation/coding pair to another may cause a large decrease in the useful bitrate and in the spectral efficiency, on account of the significant granularity.
This drawback is illustrated in FIG. 1 which represents the spectral efficiency as a function of the signal-to-noise ratio for four different modulation/coding pairs. Curve 10 corresponds to a QPSK modulation associated with a code of rate 1/3. Curve 20 corresponds to a QPSK modulation associated with a code of rate 1/2. Curve 30 corresponds to a QPSK modulation associated with a code of rate 2/3. Curve 40 corresponds to a QPSK modulation associated with a code of rate 3/4. Beyond a threshold value of signal-to-noise ratio, the spectral efficiency attains a plateau value. The maximum attainable spectral efficiency exhibits a difference of sometimes greater than 0.1 bit/sec/Hz, depending on the pairs chosen. The zones 50 of potential over-efficiency are identified on the curves. They correspond to cases where the protection added limits the useful bitrate too significantly with respect to the gain in robustness which is obtained.